Due to widespread use of broadband networks, network-based video content delivery services are expanding by using, for instance, a PC, networked TV (IPTV), or set-top box (STB) as a client terminal.
Either a unicast delivery method or a multicast delivery method is used to deliver video content through video content delivery services. In unicast delivery, a video content delivery server delivers content in accordance with a request from a client terminal on a one-to-one basis. In multicast delivery, on the other hand, the video content delivery server delivers content to many client terminals simultaneously.
In unicast or multicast delivery, RTP (Real-time Transport Protocol) is generally used because the video content delivery server has to transmit video data to a network in accordance with a video bit rate.
RTP, as the name implies, is a network protocol that excels in real-time processing required for video content delivery. However, RTP is usually implemented over UDP (User Datagram Protocol). Therefore, RTP does not permit the video content delivery server to confirm whether video data transmitted from it has arrived at a client terminal.
As such being the case, a communication protocol such as RTP Control Protocol Receiver Report (RTCP RR) is defined as described, for instance, in RFC 3550 “RTP: A Transport Protocol for Real-Time Applications,” IETF, 2003, page 42. This communication protocol measures the delivery quality of video data received by a client terminal and notifies, for instance, a video content delivery server of the measured delivery quality.
A method disclosed in JP-A-2005-223453 provides protection against spoofing, DoS attacks, unauthorized program transmission, and the like by installing a gateway at a connection point between an external IP network such as the Internet and an internal IP network such as a LAN, and constantly monitoring whether the transmission rate of data packets passing through the gateway is unduly high for a data type reported by a preceding call control packet. If the transmission rate of a data packet is higher than a predetermined value, this method discards succeeding packets or destroys analog data in the data packet to be relayed, without being discarded, to the LAN or other internal network by using a noise pattern that does not affect the listening process of a human.